A helicopter main rotor assembly is subjected to various aerodynamic, inertial, and centrifugal forces and moments during flight operations. The main rotor assembly is designed to accommodate such forces and moments through the structural and functional characteristics of the various components comprising the main rotor assembly. Of particular concern in designing a helicopter main rotor assembly are centrifugal loading (due to rotation of the main rotor blades), torsional loading (due to pilot pitch control inputs), flapwise loading (due to aerodynamically induced out-of-plane blade motions, i.e., flapping), and edgewise or chordwise loading (due to aerodynamically induced in-plane blade motions, i.e., lead or lag).
Aerodynamic drag loads and/or coriolis forces acting on the main rotor blades cause chordwise loading thereof, resulting in lead-lag motions being induced in the main rotor blades. If such induced motions are not damped, the main rotor blades may experience forced, self-excited resonant vibrations, with the attendant deleterious effects. Several types of damping devices have been utilized in articulated main rotor assemblies to dampen the induced lead-lag motions of the main rotor blades, including hydraulic dampers, elastomeric dampers, and viscous dampers.
The typical hydraulic damper embodies a piston/cylinder arrangement wherein the cylinder housing is articulately mounted to the helicopter rotor hub and the piston is articulately mounted to the root end of the main rotor blade. The induced lead-lag motion of the main rotor blade causes movement of the piston, the motion of the piston generating a pressure differential that displaces working fluid between fluid-filled chambers via a damping orifice in the piston. The energy dissipated in displacing the working fluid effectively dampens the lead-lag motion of the main rotor blade. While hydraulic dampers are attractive for maintenance accessibility and ease of repair, repair costs may be excessive due to the high failure rate of damper seals as a result of the high pressures developed in the working fluid. In addition, high tolerance machining is required to fabricate hydraulic dampers to attain acceptable damping efficiencies.
Elastomeric dampers dissipate the energy of the induced lead-lag motions of the main rotor blade by shear deformation of a plurality of elastomeric laminates. The cost of fabricating elastomeric dampers is high, however. The stiffness properties of the elastomeric laminates must be maintained constant for all elastomeric dampers utilized for a specific damping problem, e.g., the set of dampers of a helicopter main rotor assembly. Variations in stiffness properties may engender divergence of the natural in-plane rotor frequency, and lead to main rotor assembly instability. Furthermore, elastomeric dampers are often stroke limited, i.e., the strain limits of the elastomeric material forming the elastomeric laminates limits the permissible shearing motion.
Viscous dampers dissipate energy to dampen induced lead-lag motions of main rotor blades by shearing a viscous working fluid between closely spaced plates or surfaces. The energy dissipation in viscous dampers, however, causes changes in the operating temperature of the viscous working fluid. Temperature changes in the viscous working fluid, in turn, cause corresponding changes in the viscosity of the viscous working fluid. If the operating temperature range of the viscous damping fluid is large, corresponding large changes in the viscosity thereof will be experienced.
The damping force provided by a viscous damper is defined by: ##EQU1## where F is the damping force, u is the fluid viscosity, p is the fluid density, V is the shear velocity, A is the effective shear area, and c is the spacing between the shearing surfaces of the viscous damper, i.e., the shear gap. As an examination of the foregoing equation makes clear, large changes in the viscosity of the viscous working fluid result in large variations in the damping force provided by the viscous damper, i.e., the damping efficiency thereof is not constant over the operating temperature range of the viscous working fluid.
To provide viscous dampers having damping force constancy, viscous dampers have been developed wherein changes in the viscosity of the viscous working fluid are compensated by corresponding changes in at least one of the parameters of the damping force equation. For example, U.S. Pat. Nos. 3,861,503 and 2,699,846 disclose temperature-compensating viscous dampers wherein the size of the shear gap is changed to compensate for changes in viscosity of the viscous working fluid. Changes in shear gap sizing are achieved by forming the rotating member and the housing comprising the viscous damper, i.e., the shearing surfaces thereof, of materials having disparate coefficients of thermal expansion. Changes in the operating temperature of the viscous working fluid, therefore, cause disparate thermal dimensional changes in the shearing surfaces of the rotating member and the housing, such dimensional changes resulting in dimensional changes in the shear gap.
Another type of viscous damper that provides 20 temperature compensation through changes in the shear gap is disclosed in U.S. Pat. No. 3,070,192 wherein a distortable member is interposed between the damper housing and the rotating member. The distortable member, which is formed from a material having a high coefficient of thermal expansion, has the ends thereof constrained by the damper housing such that changes in the operating temperature of the viscous working fluid cause the distortable member to bow, resulting in changes in the shear gap defined between the distortable member and the rotating member.
A viscous damper that provides temperature compensation through changes in the effective shear area is described in U.S. Pat. No. 3,228,494. The shear area of the '494 viscous damper is defined by cylindrical damping surfaces of the damper housing and a damper sleeve rotatably mounted in combination with a rotating member. The damper sleeve is axially displaceable with respect to the rotating member in response to the net biasing force exerted by a helical compression spring and a temperature-dependent bi-metallic device, the constant biasing force of the compression spring acting in opposition to the temperature-dependent biasing force provided by the bi-metallic device. Axial displacement of the damper sleeve causes a change in the effective shear area defined by the cylindrical damping surfaces.
While such prior art viscous dampers tend to be relatively efficient in providing temperature compensation over the operating temperature range of the viscous working fluid, such viscous dampers are unnecessarily complicated mechanically, which reduces the reliability thereof. Moreover, mechanically complicated viscous dampers tend to be costly to fabricate, and require labor and time intensive procedures to assemble/disassemble. Furthermore, such viscous dampers are not easily adjusted to accommodate fabrication tolerance deviations, marked changes in ambient temperature conditions, and/or mechanical wear over time.